Thermomechanical fatigue of Nickel-Based Single-Crystal superalloys

Nickel-based single-crystal superalloys are extensively used in aeroengine hot-end components owing to their unique crystal structure and outstanding high-temperature mechanical properties. In this study, a thermomechanical fatigue (TMF) test platform capable of realizing precise temperature control and coordinated control of mechanical and temperature loads was constructed. The TMF of a second-generation nickel-based single-crystal superalloy (DD6) was analyzed in a temperature range of 500–950 °C at a stress ratio of 0.1 in two phases (in-phase and out-of-phase); moreover, the TMF tests were conducted at various stress levels, and the TMF life of the single-crystal specimens was determined. The OP TMF lifetime was considerably longer than the IP TMF lifetime at identical stress levels. The failure mechanism of the material in different phases was elucidated. The fracture and microstructures of the specimens were analyzed by SEM and TEM. For the IP specimens, the fracture mode was determined to be a quasi-cleavage fracture, and the γ' phase exhibited N-type rafting prior to fracture. For the OP specimens, the fracture was relatively smooth and flat. Numerous fine secondary γ' phases are precipitated in the channels of the matrix phase.